Hmm, I've had this same issue with my rodeo for the past year and a half.
It either does it only when I turn the wheel really fast and sharp to get out of a tight parking spot in a hurry or something or when I'm traveling slowly and turn sharp fast to steel a parking spot, lol.
It's a low pitch grunt/vibration while turning the wheel, doesn't affect steering performance at all though.
I can't find anything physically wrong at all nor does the fluid seem like it's bad. I haven't worried about it and it's been okay. I know it's a different vehicle and all, but it may be the exact same problem, whatever it is.
It also goes away for me too after a drive, but early in the morning when it's started in the winter it grunts.
I haven't read of any other nissan frontier owners with a similar issue, neither have I found anyone on the isuzu forums report of a similar issue so I haven't really dug into it. I guess a change of the fluid would be a good start, like truckchick78 said.